Effects of the amorphous layer on laser-induced subwavelength structures on carbon allotropes

By micro-Raman spectroscopy, we show that the structured surfaces of highly oriented pyrolytic graphite and diamond induced by 800 nm, 125 fs or 532 nm, 30 ps laser pulses are capped by thin amorphous carbon layers. Based on the results, we propose that for multiphoton ablation the thin amorphous layer with a reduced bandgap can facilitate surface ionization, raise free electron density, bring on plasmonic effects, and thus promote the growth of subwavelength structures. Therefore, concerning multipulse laser ablation of wide bandgap materials, we should take into account the effects of the superficial amorphous layer produced by preceding pulses instead of the intrinsic surface.